Method and apparatus for controlling lean operation of an internal combustion engine

Abstract

A method and system for controlling the operation of “lean-burn” internal combustion engines determines a current rate of vehicle NO x emissions, and determines a threshold value for permissible vehicle NO x emissions based on at least one current value for the intake air-fuel ratio, engine speed, engine load (e.g., brake torque, manifold air pressure, or throttle position), and/or vehicle speed. A differential NO x emissions rate is calculated as the difference between the current rate and the threshold rate, and the differential rate is accumulated over time to obtain a differential measure representing the amount by which cumulative NO x emissions have fallen below permissible levels therefor. Lean engine operation is disabled when the differential NO x emissions measure exceeds a predetermined excess vehicle NO x emission value. In this manner, vehicle NO x emissions are favorably controlled even when the engine is operated “off-cycle,” i.e., under operating conditions falling outside of the FTP driving cycles.

What is claimed is:  
     
       1. A method for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NO x , and wherein exhaust gas is directed through an exhaust gas purification system including a NO x  trap before being exhausted to the atmosphere, the method comprising: 
       determining a current rate at which NO x  is being exhausted to the atmosphere;  
       determining a threshold rate representing a maximum instantaneous rate at which NO x  is permitted to be exhausted to the atmosphere, the threshold rate being based on at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position;  
       determining a differential rate based on the current rate and the threshold rate; and  
       selecting a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio.  
     
     
       2. The method of  claim 1 , wherein determining the current rate includes sampling an output signal generated by a NO x  sensor positioned in the exhaust system downstream of the trap. 
     
     
       3. The method of  claim 1 , wherein determining the current exhaust rate includes: 
       determining a generation rate representative of the NO x  content of the exhaust being instantaneously generated by the engine; and  
       determining a storage rate representative of an instantaneous rate at which NO x  is being stored by the trap.  
     
     
       4. The method of  claim 1 , wherein the threshold rate is a function of an instantaneous value of at least one of the group consisting of the engine speed, the vehicle speed, the engine brake torque, the engine manifold air pressure, and the throttle position. 
     
     
       5. The method of  claim 4 , wherein the threshold rate is modified by a calibratable factor. 
     
     
       6. A method for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NO x , and wherein exhaust gas is directed through an exhaust gas purification system including a NO x  trap before being exhausted to the atmosphere, the method comprising: 
       determining a current rate at which NO x  is being exhausted to the atmosphere;  
       determining a threshold rate representing a maximum instantaneous rate at which NO x  is permitted to be exhausted to the atmosphere, the threshold rate being based on at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position;  
       determining a differential rate based on the current rate and the threshold rate; and  
       selecting a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio, and wherein selecting includes:  
       accumulating the differential rate over a first time period to obtain a differential measure; and  
       comparing the differential measure to an excess NO x  emissions value.  
     
     
       7. The method of  claim 6 , wherein the excess NO x  emissions value is a near-zero value. 
     
     
       8. The method of  claim 6 , including resetting the first time period to zero upon selecting an engine operating condition within the first range of engine operating conditions characterized by an air-fuel ratio that is rich of the stoichiometric air-fuel ratio. 
     
     
       9. The method of  claim 8 , further including: 
       calculating a cumulative amount of NO x  stored in the trap using the current rate; and  
       selecting the first region of engine operating conditions when the cumulative amount exceeds a trap capacity value.  
     
     
       10. The method of  claim 9 , including determining the trap capacity value as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio. 
     
     
       11. A method for controlling the NO x  emissions of a motor vehicle, wherein the vehicle is powered by an internal combustion engine capable of lean engine operation, wherein the engine generates exhaust gas including NO x , and wherein exhaust gas is directed through a three-way catalyst and a NO x  trap before being exhausted to the atmosphere, the method comprising: 
       determining a current vehicle NO x  emissions rate;  
       determining a threshold rate for vehicle NO x  emissions based on at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position;  
       determining a differential rate based on the difference between the current rate and the threshold rate;  
       accumulating the differential rate over a first time period to obtain a differential measure; and  
       disabling lean engine operation when the differential measure exceeds an excess NO x  emissions value.  
     
     
       12. The method of  claim 11 , wherein determining the current rate includes sampling an output signal generated by a NO x  sensor positioned in the exhaust system downstream of the trap. 
     
     
       13. The method of  claim 11 , wherein determining the current rate includes: 
       determining a generation rate representative of the NO x  content of the exhaust being instantaneously generated by the engine; and  
       determining a storage rate representative of an instantaneous rate at which NO x  is being stored by the trap.  
     
     
       14. The method of  claim 11 , wherein the threshold rate is a function of an instantaneous value of at least one of the group consisting of the engine speed, the vehicle speed, the engine brake torque, the engine manifold air pressure, and the throttle position. 
     
     
       15. The method of  claim 14 , wherein the threshold rate is modified by a calibratable factor. 
     
     
       16. The method of  claim 11 , including determining the total NO x  emissions value as a function of at least one of the group consisting of the engine speed, the vehicle speed, the engine brake torque, the engine manifold air pressure, and the throttle position. 
     
     
       17. The method of  claim 11 , further including: 
       determining a storage measure representing a cumulative amount of NO x  stored by the trap during the first time period based on the current rate and the threshold rate; and  
       disabling lean engine operation when the differential measure exceeds a trap capacity value.  
     
     
       18. The method of  claim 17 , wherein the trap capacity value is determined as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio. 
     
     
       19. A system for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from the engine being directed through an exhaust gas purification system including a NO x  trap before being exhausted to the atmosphere, the system comprising: 
       a controller including a microprocessor arranged to determine a current rate at which NO x  is being exhausted to the atmosphere, wherein the controller is further arranged to determine a threshold rate representing a maximum instantaneous rate at which NO x  is permitted to be exhausted to the atmosphere, the threshold rate being based on at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position; to determine a differential rate based on the current rate and the threshold rate; and to select a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio.  
     
     
       20. The system of  claim 19 , further including a NO x  sensor positioned in the exhaust system downstream of the trap, the NO x  sensor generating an output signal representative of an instantaneous concentration of NO x  in the exhaust gas, and wherein the controller is further arranged to determine the current rate by sampling the output signal generated by the NO x  sensor. 
     
     
       21. The system of  claim 19 , wherein the controller is further arranged to determine a generation rate representative of the NO x  content of the exhaust being instantaneously generated by the engine, and to determine a storage rate representative of an instantaneous rate at which NO x  is being stored by the trap. 
     
     
       22. The system of  claim 19 , wherein the controller is further arranged to determine the threshold rate as a function of an instantaneous value of at least one of the group consisting of the engine speed, the vehicle speed, the engine brake torque, the engine manifold air pressure, and the throttle position. 
     
     
       23. The system of  claim 22 , wherein the controller is further arranged to determine modify the threshold rate by a calibratable factor. 
     
     
       24. A system for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from the engine being directed through an exhaust gas purification system including a NO x  trap before being exhausted to the atmosphere, the system comprising: 
       a controller including a microprocessor arranged to determine a current rate at which NO x  is being exhausted to the atmosphere, wherein the controller is further arranged to determine a threshold rate representing a maximum instantaneous rate at which NO x  is permitted to be exhausted to the atmosphere, the threshold rate being based on at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position; to determine a differential rate based on the current rate and the threshold rate; and to select a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio,  
       wherein the controller is further arranged to accumulate the differential rate over a first time period to obtain a differential measure, and compare the differential measure to an excess NO x  emissions value.  
     
     
       25. The system of  claim 24 , wherein the excess NO x  emissions value is a near-zero value. 
     
     
       26. The system of  claim 24 , wherein the controller is further arranged to reset the first time period to zero upon selecting an engine operating condition within the first range of engine operating conditions characterized by an air-fuel ratio that is rich of the stoichiometric air-fuel ratio. 
     
     
       27. The system of  claim 19 , wherein the controller is further arranged to calculate a cumulative amount of NO x  stored in the trap using the current rate, and to select the first region of engine operating conditions when the cumulative amount exceeds a trap capacity value. 
     
     
       28. The system of  claim 27 , wherein the controller is further arranged to determine the trap capacity value as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio.